Free and open-source software (FOSS) is software that is distributed in a manner that allows its users to run the software for any purpose, to redistribute copies of it, and to examine, study, and modify, the source code. FOSS is also a loosely associated movement of multiple organizations, foundations, communities and individuals who share basic philosophical perspectives and collaborate practically, but might diverge in detail questions.

Firefox usage grew to a peak of 32% at the end of 2009, with version 3.5 overtaking Internet Explorer 7, although not Internet Explorer as a whole. Usage then declined in competition with Google Chrome. , Firefox has 9.57% usage share as a "desktop" browser, according to StatCounter, making it the second-most popular such web browser; usage across all platforms is lower at 4.66% (and then third-most popular overall). Firefox is still the most popular desktop browser in a few countries including Cuba (even most popular overall at 49.7%) and Eritrea with 72.26% and 83.28% of the market share, respectively. According to Mozilla, in December 2014, there were half a billion Firefox users around the world.